Metal Thin Film-Forming Method And Metal Thin Film

ABSTRACT

A metal thin film-forming method which comprises the step of firing metal nanoparticles each comprising a particle consisting of at least one metal selected from Ag, Au, Ni, Pd, Rh, Ru, and Pt or an alloy comprising at least two of these metals and an organic substance adhered to the periphery thereof as a dispersant, under a gas atmosphere containing water and/or an organic acid to thus form a metal thin film. This metal thin film possesses a low resistance value.

TECHNICAL FIELD

The present invention relates to a metal thin film-forming method whichmakes use of metal nanoparticles and a metal thin film.

BACKGROUND ART

There has been proposed a method for forming a conductive coating filmusing a metal colloidal solution as a method for forming an electrode ata low temperature (see, for instance, Japanese Un-Examined PatentPublication 2004-207558 (claim 1, Section Nos. 0049 to 0050)). In thiscase, the method is one for forming a conductive coating film byapplying a metal colloidal solution onto the surface of a base materialor a substrate according to the ink jet printing technique, wherein thesubstrate used in the application of the coating film is one providedthereon with a layer for receiving ink jet printing ink and the dryingstep is carried out at a temperature of not more than 100° C., after thecompletion of the coating operation. According to this method, a coatedfilm formed on a sheet of paper specially designed for the ink jetprinting as a substrate has a low volume resistivity, after the dryingof the same, on the order of 4.5×10⁻⁶Ω·cm, but a coated film formed on asheet of the usual paper for making a copy (i.e. the usual photocopyingpaper) which is free of any coating layer has a high surface resistancevalue on the order of not less than 1.0×10⁸Ω/□ (this value can beconverted into the volume resistivity of not less than 4.5×10⁷μΩ·cm onthe basis of the film thickness of 450 nm). Accordingly, the use of sucha layer for receiving ink jet printing ink should be required for thereduction of the resistance of the coated film.

In addition, there have also been proposed a reducing method (see, forinstance, Japanese Patent Application Serial No. 2003-317161 (JapaneseUn-Examined Patent Publication 2005-081501)) and the gaseous phaseevaporation technique (see, for instance, Japanese Un-Examined PatentPublication 2002-121606 (claim 6)) as methods for preparing metalnanoparticles.

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

In the methods for forming thin films which have been used asdistributing wires or electrical connections in the field of theelectric and electronic industries, the film-forming temperature usedtherein has gradually been reduced recently. In addition, as a basematerial used for forming a thin film thereon by applying metalnanoparticles onto the same, drying the applied nanoparticles and firingthe same, there have been used various kinds of base materials such asglass, polyimide, PET films, PEN films, and polycarbonate. In additionto the methods which make use of these base materials, metalnanoparticles have been sometimes applied onto a substrate or a glassplate provided thereon with TFT (thin film transistors) and accordingly,there has been desired for the reduction of the film-forming temperature(firing temperature). The firing temperature may vary depending on thecharacteristic properties of the base material selected, but it has beendesired for the firing of the same even at a temperature of not morethan 200° C. in certain cases.

In consideration of these conditions, it has strongly been desired forthe formation of a thin film having a desired thickness, while makinguse of a firing step carried out at a low temperature and reducing thenumber of coating steps or film-forming steps without using anyheat-treatment carried out at a high temperature. To this end, there hasbeen desired for the development of a method for forming a thin filmhaving a low resistivity, while making use of a dispersion of metalnanoparticles having a high metal content, without using anyheat-treatment carried out at a high temperature.

Up to now, when forming a metal nanoparticle-containing thin film usedin such applications, various problems arise such that the methodrequires the use of a high temperature firing step although theresistivity of the resulting film can be reduced; and that the methodpermits the use of a low temperature treatment, but it leads to anincrease in the number of coating steps. In this respect, if the solidcontent of the coating solution is increased by any means to reduce thenumber of coating steps, however, problems newly arise such that theresulting coating solution is quite instable and this results in theoccurrence of a secondary aggregation to thus cause the settlement ofmetal particles.

Accordingly, it is an object of the present invention to solve theforegoing problems associated with the conventional techniques and, morespecifically, to provide a method for forming a conductive metal thinfilm, using metal nanoparticles, on a substrate free of any layer forreceiving ink jet printing ink on the surface thereon, unlike such asubstrate as the paper specially designed for the ink jet printing (i.e.a substrate provided thereon with a layer for receiving the ink jetprinting ink), which is provided thereon with a thin film-receivinglayer, which can eliminate the use of any heat-treatment carried out ata high temperature, and to likewise provide a metal thin film.

Means for Solving the Problems

The metal thin film-forming method according to the present inventioncomprises the step of firing metal nanoparticles which compriseparticles consist of at least one metal selected from the groupconsisting of Ag, Au, Ni, Pd, Rh, Ru, and Pt or an alloy comprising atleast two of these metals and an organic substance adhered to theperiphery of the metal or alloy as a dispersant, wherein the firing stepis carried out under a gas atmosphere containing water or an organicacid, or both water and an organic acid. The use of such a firingatmosphere would be able to form a metal thin film having a lowresistivity value.

The foregoing organic acids are preferably saturated fatty acids orunsaturated fatty acids having not more than 4 carbon atoms. In thisconnection, if the number of carbon atoms of each organic acid exceeds4, various problems arise such that the resistance value of theresulting thin film is not reduced even when the film is fired and thatan offensive odor is generated during the firing operation.

Moreover, the metal thin film according to the present invention ischaracterized in that it is formed according to the foregoing metal thinfilm-forming method.

EFFECTS OF THE INVENTION

The present invention permits the achievement of such an effect that aconductive metal thin film having a low resistance value can be formed,using conductive metal nanoparticles, on a substrate free of any layerfor receiving ink jet printing ink on the surface thereof, unlike such asubstrate as the paper specially designed for the ink jet printing,which is provided thereon with such receiving layer, the thin film beingprepared by a heat-treatment, at a low temperature, within a gasatmosphere containing water or an organic acid, or both water and anorganic acid without using any heat-treatment carried out at a hightemperature.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, the metal constituting the metalnanoparticles used in the invention is at least one member selected fromthe group consisting of conductive metals such as Ag, Au, Ni, Pd, Rh,Ru, and Pt or an alloy comprising at least two of these metals and itmay properly be selected depending on the purposes and/or applicationsof the resulting thin film. In the following description, the term“metal” used also includes the alloys thereof. The nanoparticlesconstituted by the foregoing metals each have such a structure that anorganic substance is adhered to the circumference or periphery of eachnanoparticle as a dispersant. The term “adhered to, adhesion or thelike” herein used means that an organic substance is adsorbed on thesurface of a metal nanoparticle through a metal ion in such a mannerthat the organic substance would assist the stable dispersion of metalparticles in an organic dispersion medium.

The foregoing organic substance is at least one member selected from thegroup consisting of fatty acids and amines.

The fatty acid may preferably be at least one member selected from thegroup consisting of saturated fatty acids and unsaturated fatty acidseach having a linear or branched structure and each having 6 to 22carbon atoms. In this respect, if the fatty acid has less than 6 carbonatoms, the resulting dispersion is quite unstable and it is quite liableto undergo agglomeration and therefore, it would be impossible toincrease the metal concentration of the dispersion. On the other hand,if the number of carbon atoms present in the fatty acid molecule exceeds22, problems would arise such that the viscosity of the resultingdispersion increases and this in turn results in the reduction of thehandling properties of the dispersion when increasing the metalconcentration of the metal nanoparticle-containing dispersion and thatcarbon atoms are liable to remain in the thin film finally obtainedafter the firing step and this in turn leads to an increase in thespecific resistance value of the resulting film.

As the foregoing fatty acids, there may be listed, for instance,hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoicacid, undecanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoicacid, octadecanoic acid, eicosanoic acid, docosanoic acid,2-ethylhexanoic acid, oleic acid, linolic acid and linolenic acid.

The foregoing amine is desirably at least one member selected from thegroup consisting of aliphatic amines each having a linear or branchedstructure and each having 6 to 13 carbon atoms. In this connection, ifthe amine has less than 6 carbon atoms, the following problem arises:there is observed such a tendency that the basic properties of the amineare too strong to thus corrode metal nanoparticles and the amine mayfinally dissolve the metal nanoparticles. On the other hand, if thenumber of carbon atoms present in the main chain of the alkylamineexceeds 13, problems would arise such that the viscosity of theresulting dispersion increases and this in turn results in the reductionof the handling properties of the dispersion when increasing the metalconcentration of the metal nanoparticle-containing dispersion and thatcarbon atoms are liable to remain in the thin film finally obtainedafter the firing step and this in turn leads to an increase in thespecific resistance value of the resulting film.

The foregoing aliphatic amine is desirably a primary, secondary ortertiary alkylamine, but it may likewise be a polyvalent amine such as amonoamine, a diamine or a triamine.

Specific examples of such alkylamines include primary amines such asbutylamine, hexylamine, heptylamine, n-octylamine, nonylamine,decylamine, dodecylamine, hexa-dodecylamine, 2-ethylhexylamine,1,3-dimethyl-n-butylamine, 1-amino-undecane and 1-amino-tridecane;secondary amines such as di-n-butylamine, di-n-propylamine,di-isopropylamine, N-methylaniline, di-isobutyl-amine, di-pentylamine,and di-hexylamine; and tertiary amines such as dodecyl-dimethylamine,N,N-dibutyl-1-butaneamine, N,N-dimethyl-butylamine,N,N-dimethyl-hexylamine, and N,N-dimethyl-octylamine; as well asdiamines such as naphthalene-diamine, octamethylene-diamine, andnonane-diamine. Among these amines, preferably used herein arehexylamine, heptylamine, n-octylamine, decylamine, dodecylamine,2-ethylhexylamine, 1,3-dimethyl-n-butyl-amine, 1-amino-undecane and1-amino-tridecane.

In the present invention, the firing of the metal nanoparticles iscarried out in a gas atmosphere containing water or an organic acid, orboth water and an organic acid and the firing step is not carried out ata high temperature, but at a low temperature. This organic acid is asaturated fatty acid or unsaturated fatty acid having not more than 4carbon atoms and specific examples thereof include saturated fatty acidssuch as formic acid, acetic acid, propionic acid, n-butyric acid andiso-butyric acid; and unsaturated fatty acids such as acrylic acid,methacrylic acid, crotonic acid, isocrotonic acid, maleic acid andfumaric acid. It is sufficient in the present invention that the ratioof water to be mixed with an organic acid may range from (0 to 100):(100 to 0) as expressed in the unit of “% by mass”. In addition, the gasto be mixed with water and/or the organic acid may be air, oxygen, or aninert gas such as nitrogen gas, but the ratio of the foregoing gas isnot restricted to any specific range. According to the presentinvention, it is possible to form a metal thin film having a lowresistance value. Moreover, the firing temperature is in general notless than 50° C., preferably not less than about 80° C. and this wouldallow the formation of a thin film having a satisfactory and practicallyacceptable specific resistance value. It is sufficient to appropriatelyset the upper limit of the firing temperature at a proper level whiletaking into consideration, for instance, the kind of each particularsubstrate selected.

The method for the preparation of the metal nanoparticles is notlikewise restricted to any particular one and specific examples thereofare the reducing method such as that disclosed in Japanese PatentApplication Serial No. 2003-317161 (Japanese Un-Examined PatentPublication 2005-081501)) and the gaseous phase evaporation techniquedisclosed in Japanese Un-Examined Patent Publication 2002-121606.

For instance, such a reducing method comprises the steps of dissolvingat least one member selected from the group consisting of metalcompounds of the foregoing fatty acids and amines in a non-polar solventand then subjecting the resulting solution to a reducing treatment bythe addition of a reducing agent to thus form metal nanoparticles.

As the foregoing reducing agents, preferably used herein include, forinstance, sodium boron hydride, dimethylamine borane, and tertiarybutylamine borane. The reducing agents usable herein are not restrictedto these specific examples, but may be other known reducing agentsinsofar as they can show the same reducing action observed for theforegoing specific examples. This reducing reaction may likewise becarried out while introducing, into the reaction system, hydrogen gas,carbon monoxide gas, a hydrogen-containing gas and/or a carbon monoxidegas-containing gas.

The foregoing reducing treatment is preferably carried out under such acondition that the reaction system is subjected to a bubbling treatmentwhile stirring the same, and/or refluxing the reaction system at roomtemperature or with heating.

As has been described above, the foregoing metal compound is subjectedto a reducing treatment in a non-polar solvent to thus form metalcolloidal particles in the present invention, but impurities orcontaminants (such as boron atoms included in the reducing agent) arepresent in the reaction liquid. For this reason, deionized water isadded to the reaction liquid and then the resulting mixture is stirred,followed by allowing the mixture to stand for a predetermined timeperiod to thus recover the supernatant. Among the impurities present inthe reaction liquid, hydrophilic ones are transferred to the aqueousphase at this stage and this accordingly permits the reduction of thecontent of impurities in the reaction liquid. In this respect, a polarsolvent having the smaller number of carbon atoms may be substituted forthe deionized water. In addition, the reaction liquid treated asdescribed above can be concentrated through filtration, for instance,ultrafiltration to thus remove the excess fatty acids, fatty acid estersand/or amines and to thus increase the purity and the metalconcentration of the resulting product. As a result, a dispersion can beobtained, which comprises metal nanoparticles in a concentration of notless than 5% by mass and not more than 90% by mass.

The non-polar solvent described above and preferably used herein may be,for instance, an organic solvent whose main chain includes 6 to 18carbon atoms and which has a low polarity. If using an organic solventwhose main chain includes less than 6 carbon atoms, the polarity of thesolvent is high to ensure the formation of a desired dispersion or theresulting dispersion suffers from a problem concerning the handlingproperties and this is because, the coated layer thereof would be driedwithin an extremely short period of time. On the other hand, if thenumber of carbon atoms included in the main chain of such an organicsolvent exceeds 18, a problem arises such that carbon atoms are liableto remain in the resulting film during the firing step due to possibleincreases of, for instance, the viscosity of the liquid and/or theboiling point thereof. As such solvents, usable herein include, forinstance, long chain alkanes such as hexane, heptane, octane, decane,undecane, dodecane, tridecane and trimethyl pentane; cyclic alkanes suchas cyclohexane, cycloheptane and cyclooctane; aromatic hydrocarbons suchas benzene, toluene, xylene, trimethyl benzene and dodecyl benzene; andalcohols such as hexanol, heptanol, octanol, decanol, cyclohexanol andterpineol. These solvents may be used alone or in the form of a mixedsolvent. For instance, the solvent may be mineral spirit which is amixture of long chain alkanes.

Moreover, as the gaseous phase evaporation method, an example thereofincludes one which comprises the steps of evaporating a metal in avacuum atmosphere in the presence of the vapor of an organic solventcomprising at least one known organic solvent used for forming metalnanoparticles according to the gaseous phase evaporation method or inthe presence of a mixed vapor comprising the vapor of such an organicsolvent and the vapor of at least one member selected from the groupconsisting of, for instance, fatty acids and amines serving as adispersant to thus bring the metal vapor into close contact with thevapor of the organic solvent or the mixed vapor; cooling the resultinggaseous mixture; and recovering the metal nanoparticles to thus give aliquid containing the metal nanoparticles or a desired dispersionthereof. In this respect, when bringing the metal vapor into contactwith only the organic solvent vapor, it is also possible to obtain adesired dispersion by the addition of at least one member selected fromthe group consisting of, for instance, fatty acids and amines serving asa dispersant to the recovered liquid containing the metal nanoparticles.The method comprising the foregoing steps would permit the formation ofa metal nanoparticle-containing dispersion in which metal nanoparticleseach having a particle size of not more than 100 nm are separately orindividually dispersed.

When bringing the metal vapor into contact with only the organic solventvapor in the foregoing gaseous phase evaporation technique, a polarsolvent having a low molecular weight used for the removal of theorganic solvent may be added to the resulting metalnanoparticle-containing dispersion to thus precipitate the metalnanoparticles, after the addition of at least one member selected fromthe group consisting of, for instance, fatty acids and amines serving asa dispersant to the liquid containing the metal nanoparticles recoveredthrough cooling, followed by the removal of the supernatant to thussubstantially remove the organic solvent and the subsequent addition, tothe resulting precipitates, of at least one solvent for ensuring thesolvent-solvent exchange to thus form a dispersion in which the metalnanoparticles thus precipitated are separately or individuallydispersed. On the other hand, when bringing the metal vapor into closecontact with the mixed vapor, the polar solvent having a low molecularweight used for the removal of the organic solvent may be added to theliquid containing the metal nanoparticles recovered through cooling tothus precipitate the metal nanoparticles, followed by the removal of thesupernatant to thus substantially remove the organic solvent and thesubsequent addition, to the resulting precipitates, of at least onesolvent for ensuring the solvent-solvent exchange to thus form adispersion in which the metal nanoparticles thus precipitated areseparately or individually dispersed.

The foregoing polar solvent having a low molecular weight is a solventhaving the small number of carbon atoms and specific examples thereofpreferably used herein are methanol, ethanol and acetone.

The substrate onto which the metal nanoparticles can be appliedaccording to the method of the present invention may appropriately beselected while taking into consideration the purposes and applicationsof the resulting thin film and examples thereof include a variety ofsubstrates such as glass substrates; resin substrates made of, forinstance, polyimide, PET films, PEN films and polycarbonate; andsubstrates such as glass plates provided thereon with TFT layers. Themethods for applying a metal nanoparticle-containing dispersion ontosuch a substrate are not restricted to specific ones and they may be thespin-coating technique and the ink jet printing technique.

The present invention will hereunder be described in more specificallywith reference to the following Examples.

EXAMPLE 1

In this Example, Ag was selected as a metal material and the metalnanoparticles used herein were Ag nanoparticles comprising octanoic acidhaving 8 carbon atoms and 2-ethylhexylamine having 8 carbon atoms, bothadhered to the periphery of these Ag nanoparticles. These Agnanoparticles were dispersed in toluene and the dispersion had a metalconcentration of 40% by mass. The Ag nanoparticles used herein wereprepared according to the gaseous phase evaporation method.

This Ag nanoparticle-containing dispersion was applied onto the surfaceof a glass substrate according to the usual spin-coating technique tothus form a film, followed by firing the same at 120° C. for 30 minutesin an atmosphere formed by evaporating a water: formic acid mixed liquid[90:10 (% by mass)] in the air. After the completion of the firing step,the surface of the resulting thin film had a gloss tinged with silver.The specific resistance of the resulting film was determined using Lowresistance measurement unit (available from Mitsubishi Chemical Co.,Ltd.) and the measurement was carried out for three points on the thinfilm. In this respect, the surface resistance values were determined,and then the thickness of the thin film was determined for convertingthe same into the corresponding specific resistance. The results thusobtained are summarized in the following Table 1.

TABLE 1 Surface Resistance Film Thickness Specific Resistance (Ω/□) (μm)(Ω · cm) 0.1228 0.37 4.54 × 10⁻⁶ 0.1106 0.37 4.09 × 10⁻⁶ 0.1228 0.374.54 × 10⁻⁶

The data listed in the foregoing Table 1 clearly indicate that thespecific resistance of the resulting Ag nanoparticle-containing thinfilm is nearly equal to that observed for the pure Ag (1.59×10⁻⁶Ω·cm).

EXAMPLE 2

The same procedures used in Example 1 were repeated except that thefiring temperature was set at 80° C. to thus form a thin film and thenthe electric characteristic properties of the resulting thin film weredetermined under the same conditions used in Example 1. The results thusobtained are summarized in the following Table 2.

TABLE 2 Surface Resistance Film Thickness Specific Resistance (Ω/□) (μm)(Ω · cm) 0.2020 0.28 5.66 × 10⁻⁶ 0.1863 0.28 5.22 × 10⁻⁶ 0.1885 0.285.28 × 10⁻⁶

The data listed in the foregoing Table 2 clearly indicate that thespecific resistance of the resulting Ag nanoparticle-containing thinfilm is slightly greater than that observed for the thin film preparedin Example 1, but the thin film still has a specific resistance value onthe order of 10⁻⁶Ω·cm, even when the firing temperature was reduced.

FIG. 1 shows an SEM image illustrating the cross-sectional view of theAg thin film obtained after the firing step carried out at 80° C. andprepared in Example 2. As will be seen from this FIGURE, it can beconfirmed that particles are partially sintered in the resulting film.

The results obtained in the following Examples 3 to 18 and ComparativeExamples 1 to 12 are summarized in the following Tables 3 and 4. InExamples 3 to 18, the same procedures used in Example 1 were repeatedexcept that the metal species, dispersants and firing conditions usedwere variously changed to thus form thin films of these Examples andthen the electric characteristic properties of the resulting thin filmswere likewise determined or evaluated under the same conditions used inExample 1. In this connection, the mixing ratio of water to formic acidor acetic acid was set at a level of 90:10 (% by mass) and it wasevaporated in the air, but the concentration in the resulting atmospherewas not particularly controlled. Moreover, in Comparative Examples 1 to12, the same procedures used in Example 1 were repeated except that thefiring step was carried out in an atmosphere simply comprising the airand then the electric characteristic properties of the resulting thinfilms were likewise determined or evaluated under the same conditionsused in Example 1.

TABLE 3 Metal Conc. Ex. Metal Organic Substance (Dispersant) (% by No.Sp. Fatty acids Amines mass)  3 Ag Dodecanoic acid (C12) Octylamine (C8)40  4 Ag Decanoic acid (C10) Hexylamine (C6) 40  5 Ag Octanoic acid (C8)Dodecylamine (C12) 40  6 Ag Oleic acid (C18) Decylamine (C10) 40  7 AgDodecanoic acid (C12) Octylamine (C8) 40  8 Ag Dodecanoic acid (C12) —40  9 Ag Decanoic acid (C10) — 40 10 Ag Octanoic acid (C8) — 40 11 AgOleic acid (C18) — 40 12 Au Dodecanoic acid (C12) Octylamine (C8) 40 13Au Octanoic acid (C8) Decylamine (C10) 40 14 Au Decanoic acid (C10)Dodecylamine (C12) 40 15 Ag Octanoic acid (C8) — 40 16 Ag Oleic acid(C18) — 40 17 Au Decanoic acid (C10) Octylamine (C8) 40 18 Au Octanoicacid (C8) Dodecylamine (C12) 40 Surface Film Firing Conditions Resist-Thick- Specific Ex. Temp Time ance ness Resistance No. (° C.) (min) Atm.(Ω/□) (nm) (μΩ · cm)  3 120 30 Water + formic acid 0.26 250.00 6.40  4120 30 Water + formic acid 0.23 250.00 5.80  5 100 30 Water + aceticacid 0.28 250.00 7.00  6 130 30 Water + acetic acid 0.80 250.00 20.00  780 30 Water + formic acid 0.16 250.00 4.00  8 120 30 Water + acetic acid0.27 250.00 6.86  9 120 30 Water + formic acid 0.20 250.00 5.00 10 10030 Water + acetic acid 0.34 250.00 8.40 11 130 30 Water + acetic acid0.38 250.00 9.40 12 120 30 Water + formic acid 3.40 250.00 85.00 13 8030 Water + acetic acid 4.16 250.00 104.00 14 100 30 Water + formic acid4.40 250.00 110.00 15 100 60 Water 0.40 250.00 9.90 16 130 30 Formicacid 0.37 250.00 9.30 17 120 60 Water 3.88 250.00 97.00 18 80 30 Formicacid 5.20 250.00 130.00

TABLE 4 Metal Conc. Comp. Metal Organic Substance (Dispersant) (% by Ex.No. Sp. Fatty acids Amines mass) 1 Ag Dodecanoic acid (C12) Octylamine(C8) 40 2 Ag Decanoic acid (C10) Hexylamine (C6) 40 3 Ag Octanoic acid(C8) Decylamine (C10) 40 4 Ag Oleic acid (C18) Dodecylamine (C12) 40 5Ag Dodecanoic acid (C12) Octylamine (C8) 40 6 Ag Dodecanoic acid (C12) —40 7 Ag Decanoic acid (C10) — 40 8 Ag Octanoic acid (C8) — 40 9 Ag Oleicacid (C18) — 40 10  Au Dodecanoic acid (C12) Octylamine (C8) 40 11  AuOctanoic acid (C8) Decylamine (C10) 40 12  Au Decanoic acid (C10)Dodecylamine (C12) 40 Film Firing Conditions Surface Thick- SpecificComp. Temp Time Res. ness Res. Ex. No. (° C.) (min) Atm. (Ω/□) (nm) (μΩ· cm) 1 120 30 The atmosphere 7.20 250.00 180 2 120 30 The atmosphere6.60 250.00 165 3 100 30 The atmosphere 4.80 250.00 120 4 130 30 Theatmosphere 9780.00 250.00 244500 5 80 30 The atmosphere 5.24 250.00 1316 120 30 The atmosphere 5.04 250.00 126 7 120 30 The atmosphere 3.60250.00 90 8 100 30 The atmosphere 7.60 250.00 190 9 130 30 Theatmosphere 8.00 250.00 200 10  120 30 The atmosphere 420.00 250.00 1050011  80 30 The atmosphere 2600.00 250.00 65000 12  100 30 The atmosphere1656.00 250.00 41400

The data listed in the foregoing Tables 3 and 4 indicate that the firingin the atmosphere containing either or both of water and an organic acidcan form a thin film having a resistance value lower than that observedfor the thin film formed through the use of a firing step carried out inan atmosphere comprising the simple air, under the same temperaturecondition.

Moreover, even when using the foregoing conductive metals other than Agand Au as the foregoing metal species, and even when using dispersantsselected from the foregoing fatty acids and amines other than those usedin the foregoing Examples, one can prepare metal thin films each havinga low resistance value similar to those observed for the foregoingExamples.

INDUSTRIAL APPLICABILITY

The present invention can provide a metal thin film having a sufficientand practically acceptable specific resistance by making use of the lowtemperature firing treatment carried out under the presence of waterand/or an organic acid. Accordingly, the present invention caneffectively be used in or applied to the fields, which require theformation of metal thin films at a low temperature, for instance, in thefields of electric and electronic industries. For instance, the presentinvention can be applied to the formation of metal distributing wires ormetal electrical connections used in the fields of display machinery andtools such as flat panel display devices and in the fields of printedwirings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an SEM image illustrating the cross-sectional view of theAg thin film prepared in Example 2.

1. A metal thin film-forming method which comprises firing metalnanoparticles each comprising a particle consisting of at least onemetal selected from the group consisting of Ag, Au, Ni, Pd, Rh, Ru, andPt or an alloy comprising at least two of these metals and an organicsubstance adhered to the periphery of the metal or alloy as adispersant, wherein the firing step is carried out under a gasatmosphere containing water or an organic acid, or both water and anorganic acid.
 2. The metal thin film-forming method as set forth inclaim 1, wherein the organic acid is a saturated fatty acid or anunsaturated fatty acid having not more than 4 carbon atoms.
 3. A metalthin film characterized in that it is formed according to the metal thinfilm-forming method as set forth in claim
 1. 4. A metal thin filmcharacterized in that it is formed according to the metal thinfilm-forming method as set forth in claim 3.